The present invention relates generally to a two stage air filtration system for removing relatively large particles of materials entrained in an air stream, such as a system used in textile processing environments where large particles of waste material are generated by the textile processing equipment and carried away in an air stream.
As is well known in the textile field, various operations included in textile processing inherently tend to generate textile waste materials which vary from fine dust to relatively large particles of waste. It is common practice to operate large numbers of textile processing machines in the same general area, such as carding machines in a carding room, each of which is usually individually connected to a central waste collection system that utilizes large volumes of air flow to entrain the waste generated by the machines and carry it away from the machines. Additionally, it is common practice to recirculate the air within a textile processing area for the purpose of removing waste therefrom that could be hazardous to personnel working in the area and that could adversely affect the operation of the textile operating equipment. Additionally, this recirculated air must be "conditioned" to provide the proper operating environment for the textile machines and the textile materials being processed, such as maintaining the temperature and humidity of the recirculated air within predetermined ranges using conventional air conditioning equipment designed for this purpose.
In many textile mills, systems typically utilize large volumes of air (e.g. 50,000 c.f.m.) to entrain and carry away the waste as discussed above, and this air with entrained waste is transported through conduits to equipment designed to remove the waste from the air and return at least most of the cleaner air back to the textile processing area.
One commonly employed system used for this purpose is a two stage system that includes a first stage preseparator having a large cylindrical filter element disposed in a housing, and the air to be cleaned is introduced at the inside surface of the cylindrical filter in a direction generally tangential thereto, whereby the large waste particles in the air are removed by the filter as the air flows outwardly therethrough. These large waste particles fall to the bottom of the filter and are collected there, but the air passing through the filter, which is relatively porous, still has some fine dust entrained therein. Roughly ninety-five percent of the incoming air passes through the filter and it is then caused to flow through a panel filter, such as a panel filter of the type disclosed in U.S. Pat. No. 4,725,292 which removes essentially all of the fine dust from the air. This cleaned air is then returned directly to the main air recirculation system, and it is to be noted that this returned air is still "conditioned," at least partially, so that it does not require nearly as much "conditioning" as would drawing in ambient air from the outside, thereby rendering the system more efficient. The other roughly five percent of the original air is directed away from the upstream side of the cylindrical filter to carry with it the aforesaid relatively large waste particles collected by the filter, and this small quantity of air is introduced into a second stage filter, or fiber extractor, such as the device disclosed in U.S. Pat. No. 4,502,874. As explained in greater detail in the patent, fiber extractors of this type are designed for continuous operation and, consequently, they are large so as to include two duplicate flow paths for the air to be cleaned and valves operated by sensors for alternately directing the air through one flow path while the other flow path is taken off line and the filter is cleaned of collected waste particles, which are then allowed to drop from the housing of the fiber extractor. The cleaned air is then generally discharged to the atmosphere.
While two stage filtration systems of the foregoing type are generally satisfactory, they suffer two significant drawbacks. First, the fiber extractor itself, because it is designed for continuous operation, is relatively expensive to produce and operate primarily because of the large housing required to provide the necessary alternate flow paths for the air, and the control elements required to maintain the equipment in continuous operation. Secondly, in known systems using such fiber extractors, the large quantities of waste removed from the air is appropriately processed for disposal, such as by dumping it into a baling machine, but the cleaned air is exhausted into the atmosphere and any residual "conditioning" of such air (e.g. temperature and humidity levels) are lost. Therefore, make up air for replacing this exhausted air in the general textile environment must be drawn in from the atmosphere and passed through conditioning equipment, all of which increases the capital and operating expenses of the system as a whole because larger capacity conditioning equipment must be provided and operated.
Another known two stage filtration system employs a preseparator of the same general type as that described above, and, instead of using a fiber extractor, the small quantity of air that is diverted with the large particles of waste material is conducted through a conduit from the upstream side of the cylindrical filter of the preseparator to the inlet side of a conventional condenser. As is well known in the art, these conventional condensers include a filter screen that has a cylindrical configuration and that is rotated about its axis by a motor so that relatively large waste particles entrained in the input air stream collect on the surface of the rotating screen as a mat, and this mat is doffed or peeled off of the rotating screen by a stationary doctor blade for subsequent removal from the condenser. In this system, the cleaned air is returned to the preseparator at a point downstream of the filter thereof and recombined with the main flow of air through such filter for further processing by the panel filters as described above.
While the cleaned air is not lost to atmosphere as is the case in systems of the first type described above, this second system has its own drawbacks resulting from the use of condensers. Condensers are relatively expensive to make and operate, particularly because of the fact that it includes a rotating filter screen and because of the tolerances required to direct the air flow properly through the moving filter and to properly doff the mat from the rotating filter screen. Moreover, for much the same reasons, condensers tend to be somewhat undependable over long periods of continuous operation. Finally, condensers have relatively large operating costs resulting from the motor that is required to rotate the filter screen and from relatively large pressure drop (e.g. a maximum of about five inches of water) across the rotating filter which requires large blowers to move the air through the rotating filter.
By contrast, the two stage air filtration system of the present invention provides a system that is less expensive to fabricate and less expensive to operate, and/or that is more dependable in operation because of the simplicity of its design and operating characteristics.